1. Field of the Invention
This invention generally relates to electrical display devices.
2. Description of the Prior Art
Conventional electrical display devices usually comprise an electrode screen which is divided into a plurality of discrete electrode areas each arranged to co-operate with one or more other electrodes spaced from the electrode screen to define a plurality of display areas each capable of independent activation. For example, a typical liquid crystal display device includes two support sheets of transparent material each carrying an electrode screen and separated by a layer of liquid crystal material. The light transmission characteristics of the liquid crystal material are changed when an electrical potential is applied across two opposed electrode screens, thereby producing a pattern on the display. The electrode screens, which are deposited upon the support sheets by a printing process, are so shaped that the electrical potential can be applied selectively to adjacent areas of the display, thus allowing the pattern on the display to be changed, e.g. to represent numerals or letters. Other electrical display devices of this type include D.C. electroluminescent and vacuum fluorescent devices. In D.C. electroluminescent type displays, two electrode screens, one of which is transparent, are separated from each other by a layer of solid phosphor. The phosphor emits light when a direct current is passed through it. By suitably dividing and shaping the one electrode screen, different areas of the display can be activated separately allowing the separate areas to be viewed through the transparent electrode. In vacuum fluorescence displays, electrons are emitted from a hot-wire filament in a vacuum tube and impinge upon a fluorescent anode, separate areas of which can be activated independently.
When display devices of that kind are used to give a visual display of a variable measured value, such as speed, one electrode screen usually defines a first display area which extends along a line and is adapted to indicate a point corresponding to the magnitude of the measured or selected value. At least one second display area is also adapted for activation separately from the first display area to indicate a scale against which the magnitude of the said value can be read.
It frequently occurs that a measured value can be expressed in more than one system of units. For example, speed is commonly measured either in miles per hour or kilometers per hour; radio frequency electromagnetic radiation is measured either in meters, as a wave-length, or in kilohertz, as a frequency; and temperatures may be measured in degrees Celsius, Fahrenheit or Absolute. Alternatively, the limits of variation in the value indicated on the display may be changed, as for example, where the display is used to indicate temperature in either .degree.C. or .degree.F., or forms part of an instrument capable of measuring more than one type of value, such as volts, amperes and resistance. If an electrical display device of the type to which this invention relates is to be used for such applications, a plurality of separate display areas must be provided each adapted to indicate a respective scale of units against which the magnitude of the value can be read. Each of the electrode areas must also be provided with its own leads in the form of strips of conductive material which connects the electrode area to a separate terminal. The electrode area must also be positioned adjacent the other display areas but without overlapping therewith. Where separate display areas are used to indicate separate scales, the positioning of the leads in a non-contacting configuration presents a major problem in the design of the display.